Credit due not passed

In December i upgraded my contract to a CHEAPER plan, the person as the store did not check that this has pulled through properly in the system, and i was still billed for the more expensive amount. Literally nobody knew how to fix this, i was sent from here to there getting a different answer. I finaly got hold of Marc Quin, Regional manager and after payment the old amount for 3 month go an email to confirm that a credit will be passed for the month of March. Suprize surprize my bill came and NO CREDIT. I have waited and wasted my time enough now. MTN is very quick to deduct the wrong amounts, but now that i need to get my credit that is due to me, i have to wait and battle. I WILL NOT renew my contracts at the end of this term. Because i cannot battle with the same issue for another six months after upgrading.